The invention relates to a polymerization initiator comprising an alkaline ear th metal compound chosen from the group
a) of heteroleptic alkaline earth metal compound s of the formula I
Lxe2x80x94Mxe2x80x94Rxe2x80x83xe2x80x83(I)
xe2x80x83or
b) of cationic alkaline earth metal complexes of the formula II
[Dxe2x86x92Mxe2x80x94R]+Xxe2x88x92xe2x80x83xe2x80x83(II),
xe2x80x83where
M: is Ca, Sr or Ba,
L: is a polymerization-inactive ligand,
R: is a polymerization-active ligand,
D: is a donor ligand, and
X: is a non-coordinating anion.
The invention further relates to processes for the preparation of the polymerization initiators and processes for anionic polymerization in the presence of the polymerization initiator.
Styrene can be polymerized either free-radically, anionically, cationically or in the presence of metallocene catalysts. Free-radical polymerization initiated thermally or by means of peroxides produces atactic polystyrene.
The preparation of syndiotactic polystyrene in the presence of metallocene catalyst system is known and is described, for example, in detail in EP-A-0 535 582. Because of its crystallinity, syndiotactic polystyrene has a very high melting point of about 270xc2x0 C., high rigidity and tensile strength, dimensional stability, a low dielectric constant and high chemical resistance. The profile of mechanical properties is retained even at temperatures above the glass transition temperature.
Isotactic polystyrene, obtainable by means of catalysts comprising titanium tetrachloride and alkylaluminum chlorides, has been described by G. Natta et al. in Journal of the American Chemical Society 77 (1955), page 1708. Isotactic polystyrene is crystalline and has a melting point of 240xc2x0 C. Because of the very slow rate of crystallization, it is unsuitable for industrial applications, e.g. for injection molding.
Anionic and cationic polymerization, like free-radical polymerization, usually also leads to atactic polystyrene. Anionic polymerization has living character and therefore several advantages over free-radical polymerization or polymerization catalyzed by metallocenes. Thus, for example, it is possible to control simply the molecular weight via the ratio of initiator to monomers and the formation of block copolymers. The polymers prepared by the anionic process have a narrow molecular weight distribution and low residual monomer contents.
The anionic polymerization of styrene and butadiene is usually initiated by organolithium polymerization initiators. The anionic polymerization initiation by organobarium compounds is known, for example, from U.S. Pat. Nos. 3,965,080, 4,012,336. The unpublished DE-A 197 54 504 describes an improved process for the preparation of bisorganoalkaline earth metal compounds.
Russian Chemical Reviews, Vol. 50, 1981, p. 601-614 gives an overview of the synthesis methods and the use of organoalkaline earth metals in the anionic polymerization of unsaturated monomers. Some of the known syntheses for organoalkaline earth metals are complex or produce the desired compounds in low yields or contaminated with byproducts.
B. Nakhmanovich et al., Journal of Makromol. Science Chem. A9(4), pages 575 to 596 (1975) describe the random copolymerization of styrene and butadiene with a high cis 1,4-content of the butadiene units.
The stereoselective polymerization of styrene using anionic polymerization initiators has hitherto not been described.
The heteroleptic alkaline earth metal compounds known from Tesh et al. Journal of the American Chemical Society 116 (1994), page 2409 to 2417, and Burkey et al. Organometallics 13 (1994), page 2773 to 2786 are not polymerization-active.
It is an object of the present invention to provide an anionic polymerization initiator which is also stereoselective with regard to the polymerization of styrene. The polymerization initiator was to combine the advantages of anionic and of metallocene-catalyzed styrene polymerization.
It is a further object of the invention to provide a simple and favorable process for the preparation of the polymerization initiators.
We have found that the first object is achieved by a polymerization initiator comprising an alkaline earth metal compound chosen from the group
a) of heteroleptic alkaline earth metal compounds of the formula I
Lxe2x80x94Mxe2x80x94Rxe2x80x83xe2x80x83(I)
xe2x80x83or
b) of cationic alkaline earth metal complexes of the formula II
[Dxe2x86x92M-R]+Xxe2x88x92xe2x80x83xe2x80x83(II),
xe2x80x83where
M: is Ca, Sr or Ba,
L: is a polymerization-inactive ligand,
R: is a polymerization-active ligand,
D: is a donor ligand, and
X: is a non-coordinating anion.
Where appropriate, solvents coordinated to the alkaline earth metal M, such as tetrahydrofuran, or groups carrying heteroatoms in the ligand R are not shown in the formulae (I+II) for simplicity.
We have also found that the second object is achieved by a process for the preparation of the polymerization initiators and processes for anionic polymerization in the presence of the polymerization initiator.
The term heteroleptic alkaline earth metal compound is used to describe one with two different ligands on the alkaline earth metal.
A suitable polymerization initiator is also a mixture of two or more alkaline earth metal compounds of the formula I or II.
The polymerization initiators can additionally comprise alkaline earth metal compounds of the formulae III and IV:
Lxe2x80x94Mxe2x80x94Lxe2x80x83xe2x80x83(III)
Rxe2x80x94Mxe2x80x94Rxe2x80x83xe2x80x83(IV),
where L, M and R are as defined in claim 1.
For the stereoselective anionic polymerization, heteroleptic alkaline earth metal compounds are necessary which have a polymerization-active radical and a second, sterically directing ligand. As a result of ligand exchange, these compounds are in Schlenk equilibrium with the respective homoleptic complexes:
Lxe2x80x94Mxe2x80x94L+Rxe2x80x94Mxe2x80x94R⇄Lxe2x80x94Mxe2x80x94R
The position of the Schlenk equilibrium can be determined using the following equation:   K  =                    [                  L          -          M          -          R                ]            2                      [                                            (              L              )                        2                    ⁢          M                ]            *              [                                            (              R              )                        2                    ⁢          M                ]            
Complexes suitable for the stereoselective polymerization must have a Schlenk equilibrium which is predominantly on the side of the heteroleptic complexes and in which the ligand exchange proceeds slowly in relation to the polymerization.
For a stereoselective polymerization, it is advantageous for the proportion of alkaline earth metal compounds of the formula IV to be at most 10 mol %, preferably 0 to 1 mol %, based on the total of all alkaline earth metal compounds.
The polymerization-inactive ligand L generally has a lower basicity than polystyryllithium. The pKa value of the ligand is preferably below 30, in particular below 20.
Examples of suitable polymerization-inactive ligands L are cyclic or open-chain hydrocarbons having a delocalized electron system in which one or more CH fragments can be replaced by isoelectronic fragments, sterically hindered hydrocarbons bonded by heteroatoms, or clusters bonded by a halogen.
Examples of cyclic hydrocarbons having a delocalized electron system are unsubstituted or mono- or polysubstituted cyclopentadienyls, indenyls, fluorenyls, fulvalenediyls or hydropentalenyls. The substituents can be alkyls, preferably C1- to C10-alkyl, 5- to 7-membered cycloalkyl, which for its part can carry a C1 to C10-alkyl as substituent, C6- to C15-aryl or arylalkyl. Other suitable substituents are groups with heteroatoms such as silanes, amines, phosphanes, arsanes, stilbanes. Preferred heteroatom-carrying groups as substituents are trialkylsilyls, in particular trimethylsilyl.
One or more CH fragments of the cyclic hydrocarbons can also be released by isoelectronic N or P or S fragments, e.g. pyrrolyl anion C4H4Nxe2x88x92, phosphacyclopentadiene C4H4Pxe2x88x92 or arsacyclopentadiene C4H4Asxe2x88x92. Suitable polymerization-inactive ligands are also anionic boron heterocycles such as diborolenyl or borinate. Preferred cyclic hydrocarbons are cyclopentadienyl, methylcyclopentadienyl, ethylcylopentadienyl, n-butylcyclopentadienyl, pentamethylcyclopentadienyl, trimethylsilylcyclopentadienyl, indenyl and fluorenyl.
Two identical or different cyclic hydrocarbons can also be bridged via an alkyl, silyl or phosphoryl group. Preferred bridged cyclic hydrocarbons having a delocalized electron system are (dimethylphenylsilyl)tetramethylcyclopentadienyl.
Examples of open-chain hydrocarbons having a delocalized electron system are anionic 3-, 5- or 7-membered hydrocarbons in which optionally one or more CH2 fragments can be replaced by isoelectronic NR or O, and a CH fragment can be replaced by N or R2P. Preferably, pentadienyl or diphenylalkyls are suitable. Particular preference is given to bis(4-methylbenzylide)diphenylphosphonium. Sterically hindered hydrocarbons bonded via heteroatoms, as polymerization-inactive ligand L, are, for example, sterically hindered alkoxides or phenoxides, amides, sulfides or phosphides. Suitable sterically hindered radicals are tert-butyl, dimethylphenyl, di-tert-butylphenyl, diphenylphenyl. Suitable as polymerization-active ligand R are ligands with a pKa value above 30, preferably above 40. Preferred polymerization-active ligands are carbanions, for example of the formula R1R2R3Cxe2x88x92 or R1(R2R3C)Cxe2x88x92, where R1, R2 and R3 can be identical or different radicals and are each hydrogen, alkyls such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, hexyl, heptyl, octyl, nonyl, dodecyl, octadecyl, aryls, such as phenyl or substituted phenyls, such as 3,5-dimethylphenyl, p-t-butylphenyl, p-octylphenyl, p-dodecylphenyl, o-, m-, p-tolyl, biphenyl, naphthyl, arylalkyls such as benzyl, phenylethyl, 2-phenylpropyl, 6-phenylhexyl, p-methylphenylethyl, p-t-amylbenzyl, alkenyls, such as vinyl, allyl, 3-butenyl, 4-hexenyl, cycloalkyls such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclooctyl, cycloalkenyls such as cyclohexenyl or cyclooctenyl. The radicals R1, R2 and R3 can, however, also be NR1R2, OR1, SiR1R2R3, SR1, PR1R2, where the radicals R1, R2, R3 bonded to the heteroatom are as defined above. The radicals R1, R2 or R3 can also be bonded cyclically with one another and form cycloaliphatic or aromatic rings. The polymerization-active ligands R can also carry functional groups which are inert to the metal-carbon bond. Examples of inert groups are trimethylsilyl, trimethylsiloxy, ether, dialkylamino or cycloalkylamino groups. Preferred polymerization-active ligands are unsubstituted or substituted benzyl, bis(trimethylsilyl)methyl, o,o-dialkylphenyl, allyl or alkenyl.
The polymerization-active ligands R can also be R1R2Nxe2x88x92 or R1R2R3Sixe2x88x92, where R1, R2 and R3 are as defined above.
The cationic alkaline earth metal complexes of the formula II contain a mono- or polydentate donor ligand D. Examples thereof are mono- or polydentate ethers, thioethers, amines or sulfides. Preference is given to chelate ligands having two or more oxygen or nitrogen atoms, such as pentamethyldiethylentriamine (PMDTA).
Suitable as non-coordinating anion X in formula II are, in principle, all abovementioned polymerization-inactive ligands in the form Lxe2x88x92 which have been displaced by the donor ligand D. Other suitable non-coordinating anions are, for example, unsubstituted and halogen- or haloalkyl-substituted tetraphenylborates or carboranates. Examples thereof are tetraphenyl borate, tetra(p-tolyl) borate, tetra(o-tolyl) borate, tetra(o,p-dimethylphenyl) borate, tetra(m,m-dimethylphenyl) borate, tetra(pentafluorophenyl) borate and tetra(p-trifluoromethylphenyl) borate.
The heteroleptic alkaline earth metal compounds can, for example, be prepared by reaction of an alkaline earth metal compound of the formula R2M with a compound of the formula LH or of the formula L2M, where M, R and L are as defined in claim 1, and H is hydrogen. In the reaction of an alkaline earth metal compound of the formula R2M with a compound of the formula L2M, the heteroleptic compound formed is in Schlenk equilibrium with the starting compounds. The equilibrium can be controlled by the choice of ligand L.
Preferred alkaline earth metal compounds of the formula R2M are dibenzylbarium, bis(diphenylmethyl)barium, bis(1,1,3-triphenylpropyl)barium and bis[(2-dimethylaminophenyl)(trimethylsilyl)methyl]calcium. To prepare the heteroleptic alkaline earth metal compounds according to the invention, they are preferably reacted with dimethylphenyl(tetramethylcyclopentadienyl)silane, (diethylamino)dimethyl(tetramethylcyclopentadienyl)silane or 9-trimethylsilylfluorene, as compound of the formula LH, or with bis[(dimethylphenylsilyl)tetramethylcyclopentadienyl)]barium, bis[(diethylaminodimethylsilyl)tetramethylcyclopentadienyl)]barium, bis[bis(4-methylbenzylide)diphenylphosphonium]barium, bis(pentamethylcyclopentadienyl)barium, bis[(dimethylphenylsilyl)fluoren-9-yl]barium or bis[(dimethylphenylsilyl)tetramethylcyclopentadienyl]calcium, as preferred alkaline earth metal compounds of the formula L2M.
The polymerization initiators according to the invention are suitable for the polymerization of anionically polymerizable monomers, in particular of vinylaromatic monomers or dienes, but also of acrylates, methacrylates, acrylonitriles or vinyl chloride. For this purpose, the polymerization initiators according to the invention are generally used in amounts in the range from 0.001 to 5 mol %, based on the monomers to be polymerized.
Preferred dienes are butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene, 1,3-hexadienes or piperylene or mixtures thereof.
Vinylaromatic monomers which can be used are, for example, styrene, xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene or 1,1-diphenylethylene or mixtures thereof.
Particular preference is given to using butadiene and styrene.
The polymerization is expediently carried out in an aliphatic or aromatic hydrocarbon or hydrocarbon mixture, preferably in benzene, toluene, ethylbenzene, xylene, cumene or cyclohexane. Particular preference is given to using cyclohexane or toluene. Further process parameters are unimportant for carrying out the process. It is possible to operate in the temperature and pressure ranges known for the anionic polymerization of butadiene and styrene.
The polymerization initiators according to the invention can be used to prepare styrene polymers with high syndiotacticity and narrow molar mass distributions. For example, it is possible to achieve a polydispersity D of at most 2.5, preferably at most 2.
Because of the living character, the polymerization initiators according to the invention can be used to prepare, by sequential monomer addition, block copolymers of varying structure.
The polymerization initiators according to the invention can thus also be used to prepare block copolymers with syndiotactic blocks of vinylaromatic monomers, for example styrene-butadiene-styrene three-block copolymers, which, depending on the butadiene content, are suitable as transparent, impact-resistant polystyrene or thermoplastic elastomers with increased heat deflection temperature.
Where a vinylaromatic monomer and diene are present at the same time, it is possible to prepare random copolymers with a high content of 1,4 linkages or a low content of 1,2 linkages of the diene monomers.